One pervasive feature of daily life in modern industrialized societies is the use of disposable products, particularly disposable products made of paper. Paper towels, facial tissues, sanitary tissues, and the like are in almost constant use. Naturally, the manufacture of items in such great demand has become, in the Twentieth Century, one of the largest industries in industrially developed countries. The general demand for disposable paper products has, also naturally, created a demand for improved versions of the products and of the methods of their manufacture. Despite great strides in paper making, research and development efforts continue to be aimed at improving both the products and their processes of manufacture.
Disposable products such as paper towels, facial tissues, sanitary tissues, and the like are made from one or more webs of tissue paper. If the products are to perform their intended tasks and to find wide acceptance, they, and the tissue paper webs from which they are made, must exhibit certain physical characteristics. Among the more important of these characteristics are strength, softness, and absorbency.
Strength is the ability of a paper web to retain its physical integrity during use.
Softness is the pleasing tactile sensation consumers perceive when they crumple the paper in their hands and when they use the paper for its intended purposes.
Absorbency is the characteristic of the paper which allows it to take up and retain fluids, particularly water and aqueous solutions and suspensions. In evaluating the absorbency of paper, not only is the absolute quantity of fluid a given amount of paper will hold significant, but the rate at which the paper will absorb the fluid is also important. In addition, when the paper is formed into a device such as a towel or wipe, the ability of the paper to cause a fluid to be taken up into the paper and thereby leave a dry wiped surface is also important.
Processes for the manufacturing of disposable paper products for use in tissue, toweling and sanitary products generally involve the preparation of an aqueous slurry of paper fibers and then subsequently removing the water from the slurry while contemporaneously rearranging the fibers in the slurry to form a paper web. Various types of machinery can be employed to assist in the dewatering process. Currently, most manufacturing processes employ machines which are known as Fourdrinier wire papermaking machines or machines which are known as twin (Fourdrinier) wire papermachines. In Fourdrinier wire papermaking machines, the paper slurry is fed onto the top surface of a traveling endless belt, which serves as the initial papermaking surface of the machine. In twin wire machines, the slurry is deposited between a pair of converging Fourdrinier wires in which the initial dewatering and rearranging in the papermaking process are carried out. After the initial forming of the paper web on the Fourdrinier wire or wires, both types of machines generally carry the paper web through a drying process or processes on another fabric in the form of an endless belt which is often different from the Fourdrinier wire or wires. This other fabric is sometimes referred to as a drying fabric. Numerous arrangements of the Fourdrinier wire(s) and the drying fabric(s) as well as the drying process(es) have been used successfully and somewhat less than successfully. The drying process(es) can involve mechanical compaction of the paper web, vacuum dewatering, drying by blowing heated air through the paper web, and other types of drying processes.
As seen above, papermaking belts or fabrics carry various names depending on their intended use. Fourdrinier wires, also known as Fourdrinier belts, forming wires, or forming fabrics are those which are used in the initial forming zone of the papermaking machine. Dryer fabrics as noted above, are those which carry the paper web through the drying operation of the papermaking machine. Various other types of belts or fabrics are possible also. Most papermaking belts employed in the past are commonly formed from a length of woven fabric the ends of which have been joined together in a seam to form an endless belt. Woven papermaking fabrics generally comprise a plurality of spaced longitudinal warp threads and a plurality of spaced transverse weft threads which have been woven together in a specific weaving pattern. Prior belts have included single layer (of warp and weft threads) fabrics, multilayered fabrics, and fabrics with several layers of interwoven warp and weft threads. Initially, the threads of papermaking fabrics were made from wires comprised of materials such as bronze, stainless steel, brass or combinations thereof. Often various materials were placed on top of and affixed to the fabrics in an attempt to make the dewatering process more efficient. Recently, in the papermaking field, it has been found that synthetic materials may be used in whole or part to produce the underlying wire structures, which would be superior in quality to the forming wires made of metal threads. Such synthetic materials have included nylon, polyesters, acrylic fibers and copolymers. While many different processes, fabrics, and arrangements of these fabrics have been used, only certain of these processes, fabrics, and arrangements of these fabrics have resulted in commercially successful paper products.
An example of paper webs which have been widely accepted by the consuming public is the webs made by the process described in U.S. Pat. No. 3,301,746, Sanford and Sisson, issued Jan. 31, 1967. Other widely accepted paper products are made by the process described in U.S. Pat. No. 3,994,771, Morgan and Rich, issued Nov. 30, 1976. Despite the high quality of products made by these two processes, however, the search for still improved products has, as noted above, continued.
Another commercially significant improvement was made upon the above paper webs by the process described in U.S. Pat. No. 4,529,480, Trokhan, issued July 16, 1985. The improvement included utilizing a papermaking belt (termed a "deflection member") which was comprised of a foraminous woven member surrounded by a hardened photosensitive resin framework. The resin framework was provided with a plurality of discrete, isolated, channels known as "deflection conduits". The process in which this deflection member was used involved, among other steps, associating an embryonic web of papermaking fibers with the top surface of the deflection member and applying a vacuum or other fluid pressure differential to the web from the backside (machine-contacting side) of the deflection member. The papermaking belt used in this process was termed a "deflection member" because the papermaking fibers would be deflected into and rearranged into the deflection conduits of the hardened resin framework upon the application of the fluid pressure differential. The deflection member was made according to the process described in U.S. Pat. No. 4,514,345, Johnson et al., issued Apr. 30, 1985. This process included the steps of: 1 ) coating the foraminous woven element with a photosensitive resin; 2) controlling the thickness of the photosensitive resin to a pre-selected value; 3) exposing the resin to a light having an activated wave length through a mask having opaque and transparent regions; and 4) removing the uncured resin. By utilizing the aforementioned improved papermaking process, it was finally possible to create paper having certain desired pre-selected characteristics. The paper produced using the process disclosed in U.S. Pat. No. 4,529,480 is characterized by having two physically distinct regions distributed across its surface; one is a continuous network region which has a relatively high density and high intrinsic strength, the other is a region which is comprised of a plurality of domes which have relatively low densities and relatively low intrinsic strengths (when compared to the network region), which are completely encircled by the network region.
The paper produced by the aforementioned process was actually stronger, softer, and more absorbent than the paper produced by the preceding processes as a result of several factors. The strength of the paper produced was increased as a result of the relatively high intrinsic strength provided by the network region. The softness of the paper produced was increased as a result of the provision of the plurality of low density domes across the surface of the paper. The absorbency of the paper was increased due to the fact that the paper had a generally lower density, whereas the rate of absorbency was increased because the network was able to distribute absorbed liquids to the absorbent low density domes in an orderly fashion.
Although the aforementioned improved process worked quite well, it has been found that the hardened photosensitive polymeric resin contained in the papermaking belt rapidly degrades with time resulting in the belts failing prematurely. The principle degradation mechanism for these deflection members (papermaking belts) is oxidation of the photopolymer resin. To retard this, it is necessary to add antioxidant chemicals, such as high molecular weight hindered phenols, to the liquid photopolymer resin prior to final polymerization by light of an activating wave length (e.g., UV light). However, there is an upper limit to the amount of these chemicals that can be included in the liquid resin for three reasons: (a) these chemicals have a negative impact on the photospeed (reaction rate) of the resin, (b) solubility limitations of the chemicals in the resin, and (c) the resin structure is weakened by displacement of the polymer. While running on a paper machine, these materials are consumed and/or removed as they protect against oxidation. As the antioxidant content is lowered or eliminated, the resin becomes vulnerable to degradation and the belt is soon destroyed. Thus, a need exists for a method of replacing the chemical compounds which are depleted from the belt during the papermaking operation.
The present invention pertains to a process for improving the useful belt life through the continuous application of chemical compounds to the solid polymeric resin containing belts during the papermaking operation. In particular, by adding appropriate antioxidant chemicals to the belt during the papermaking operation, the belt life can be greatly extended. This technique overcomes the current limitation on the amount of antioxidants that can be added to the unpolymerized liquid resin. It also counteracts the natural depletion of antioxidant in the resin during normal papermaking operations.
It is an object of this invention to provide a process for extending the operating life of papermaking belts containing a solid polymeric photosensitive resin through the continuous application of an effective amount of a chemical compound to the papermaking belt during the papermaking operation.
It is another object of the present invention to provide a process for the continuous application of low levels of antioxidant chemicals to the paper-contacting surface of these resin containing papermaking belts during their use; thereby protecting the resin against oxidation.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.